Providing a reliable operating system for clients of a net-booted environment

ABSTRACT

A method and apparatus are provided for supplying a reliable and maintainable operating system in a net-booted environment. According to one embodiment, a network computer (NC) client boots from a boot image provided by an NC server. The boot image includes information identifying the location of one or more system volumes on the NC server that contain operating system software. In response to an attempt to modify the contents of the one or more system volumes, the NC client causes information identifying the modification to be recorded on the NC server separate from the one or more system volumes in a storage area associated with the NC client.

This application is a continuation of U.S. patent application Ser. No.10/763,581, filed on Jan. 23, 2004 now U.S. Pat. No. 7,233,985, which isa continuation of U.S. patent application Ser. No. 09/420,614, filed onOct. 18, 1999, now issued as U.S. Pat. No. 6,751,658.

COPYRIGHT NOTICE

Contained herein is material that is subject to copyright protection.The copyright owner has no objection to the facsimile reproduction ofthe patent disclosure by any person as it appears in the Patent andTrademark Office patent files or records, but otherwise reserves allrights to the copyright whatsoever.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to network computing. More particularly,the invention relates to the provision, administration, and maintenanceof an operating system in a net-booted environment.

2. Description of the Related Art

Most organizations currently employ local area networks (LANs) of thickclients, e.g., personal computers. While this represents an improvementover the disconnected computing environments of a decade earlier, manylimitations still exist. In current LAN environments, each clientcomputer has its own local copy of operating system software,application programs, and user customizations to the desktopenvironment. Typically there is no centralized mechanism for maintaininga consistent system configuration in such a computing environment.Consequently, individual user workstations often get out-of-sync witheach other as one or more users upgrade to newer versions of theoperating system, upgrade their application programs, or installapplication programs that were not part of the original systemconfiguration. Additionally, in this type of uncontrolled, decentralizedenvironment, the operating system of a client computer can easily becomecorrupted. This is especially true with the Microsoft® Windows® 95, 98and NT operating systems where user modification of a single system filecan have undesirable consequences and require significant downtime. Forexample, editing the Windows Registry file could render a clientcomputer unusable thereby requiring reinstallation of the computer'soperating system software and all the application programs.

In view of the foregoing, it should be apparent that administration andmaintenance of current computing environments is complex and timeconsuming. Therefore, what is needed is a reliable computing environmentthat can be maintained more easily and at a lower cost.

BRIEF SUMMARY OF THE INVENTION

A method and apparatus are described for providing a reliable andmaintainable operating system in a net-booted environment. According toone embodiment, a network computer (NC) client boots from a boot imageprovided by an NC server. The boot image includes informationidentifying the location of one or more system volumes on the NC serverthat contain operating system software. In response to an attempt tomodify the contents of the one or more system volumes, the NC clientcauses information identifying the modification to be recorded on the NCserver separate from the one or more system volumes in a storage areaassociated with the NC client.

Other features and advantages of the invention will be apparent from theaccompanying drawings and from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a simplified block diagram that conceptually illustrates anetwork computer system according to one embodiment of the presentinvention.

FIG. 2 is a block diagram of a digital processing system which may beused in accordance with one embodiment of the present invention.

FIG. 3 is a flow diagram illustrating processing that occurs on an NCclient and an NC server to accomplish a net-boot of the NC clientaccording to one embodiment of the present invention.

FIG. 4 is a flow diagram illustrating how NC client write requestsdirected to the system volume are handled according to one embodiment ofthe present invention.

FIG. 5 is a flow diagram illustrating how NC client read requestsdirected to the system volume are handled according to one embodiment ofthe present invention.

FIG. 6 is a simplified block diagram illustrating an exemplaryhierarchical directory structure that may be used by an NC serveraccording to one embodiment of the present invention.

FIG. 7 is a flow diagram illustrating NC system administrationprocessing according to one embodiment of the present invention.

FIG. 8 is a simplified block diagram illustrating two NC systems coupledvia the Internet.

FIG. 9 is a block diagram that conceptually illustrates NC clientinteraction with a SplitOS according to one embodiment of the presentinvention.

FIG. 10 is a block diagram that conceptually illustrates the structureof a shadow system volume according to one embodiment of the presentinvention.

FIG. 11 is a block diagram that conceptually illustrates the structureof a shadow system volume according to another embodiment of the presentinvention in which a banding feature is employed.

FIG. 12 is an example of a machine-readable medium that may be accessedby a digital processing system, such as an NC client or an NC server,according to one embodiment of the present invention.

DETAILED DESCRIPTION

A method and apparatus are described for providing a reliable andmaintainable operating system in a net-booted environment. Broadlystated, embodiments of the present invention seek to provide afault-tolerant, self-repairing, and remotely maintainable operatingsystem for the clients in a net-booted environment. According to oneembodiment of the present invention, a network computer (NC) systemmaintains a copy of the operating system that cannot be corrupted byordinary users of the NC system. Additionally, the NC system maypreserve user customizations, such as preferences, by maintainingindividual, user, storage areas. When an NC client boots from thenetwork and accesses a stored copy of the operating system from an NCserver, the user's preferences are dynamically merged with the systemenvironment provided to the NC client. Advantageously, since, the user'sdesktop preferences and other customized settings are all preserved fromsession to session and supplied to the NC client as it boots from thenetwork, the user may login to any NC client on the network and have thesame user experience. According to another embodiment, a networkadministrator can upgrade every NC client in the NC system to a newversion of the operating system by simply replacing a single file on theNC server. Further, according to another feature of the presentinvention, the network administrator can perform such an upgraderemotely from any NC client of the NC system. Advantageously, in thismanner, NC system administration and maintenance costs are kept low ascompared to a typical network of thick clients that each has a localcopy of the operating system that must be replaced.

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout some of these specific details. In other instances, well-knownstructures and devices are shown in block diagram form.

The present invention includes various steps, which will be describedbelow. The steps of the present invention may be performed by hardwarecomponents or may be embodied in machine-executable instructions, whichmay be used to cause a general-purpose or special-purpose processor orlogic circuits programmed with the instructions to perform the steps.Alternatively, the steps may be performed by a combination of hardwareand software.

The present invention may be provided as a computer program productwhich may include a machine-readable medium having stored thereoninstructions which may be used to program a computer (or otherelectronic devices) to perform a process according to the presentinvention. The machine-readable medium may include, but is not limitedto, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks,ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, flash memory, orother type of media/machine-readable medium suitable for storingelectronic instructions. Moreover, the present invention may also bedownloaded as a computer program product, wherein the program may betransferred from a remote computer (e.g., a server) to a requestingcomputer (e.g., a client) by way of data signals embodied in a carrierwave or other propagation medium via a communication link (e.g., a modemor network connection). Accordingly, herein, a carrier wave shall beregarded as comprising a machine-readable medium.

Importantly, while embodiments of the present invention will bedescribed with reference to the Mac® operating system (OS) software andNetBoot technology on a Mac OS X Server, the method and apparatusdescribed herein are equally applicable to other types of servers andoperating systems, such as Microsoft® Windows® 95/98, Windows NT,Windows 2000, Windows CE, AIX, UNIX®, Linux, and the like. Consequently,the techniques described herein are thought to be generally useful inconnection with remote bootstrapping of client systems in a variety ofoperating environments and networking environments.

Terminology

Before describing an exemplary environment in which various embodimentsof the present invention may be implemented, some terms that will beused throughout this application will briefly be defined.

The terms “boot,” “booting,” “bootstrap,” and “bootstrapping” generallyrefer to the process of starting-up a computer, which typically involvesloading operating system software.

An “image” refers to an electronic representation of an individual diskor a portion thereof.

The term “file system” generally refers to the logical structures andsoftware routines used to control access to and from the storage on astorage medium, such as a hard disk system.

An “operating system” refers to a program that manages a computer'sinternal functions and provides a means to control the computer'soperations and file system. Examples of operating systems include:MacOS, Windows 95/98, Windows NT, Windows 2000, Windows CE, AIX, UNIX,and Linux.

“Personal computer” generally refers to a computer system equipped withsystem, utility, application software, and/or input/output devices.Importantly, as used herein, the term “personal computer” is intended torefer collectively and individually to stand-alone Macintosh® computers,Apple® computers, IBM® Personal Computers, IBM® PC-compatible computers,Amiga computers, and others, such as Commodore that are no longermanufactured and those yet to be manufactured.

The term “thick client” generally refers to a computer system, such as apersonal computer (PC), that includes a hard drive or other localstorage medium. Traditionally, such computer systems boot and executeapplication programs from the local storage medium.

A “thin client” generally refers to a disk-less computer system thatrelies upon external means, such as a server, to supply it with anoperating system and application programs.

As used herein, the terms “network computer client” or “NC client”generally refer to a client (thick or thin) that boots by accessing acopy of the operating system over a network. As such, an NC client isnot required to be disk-less.

The terms “network computer server” or “NC server” generally refer to acomputer system that services requests of NC clients. For example, an NCserver may provide access to a copy of the operating system to an NCclient in response to a boot request.

A “network computer system” or “NC system” refers to a network includingone or more NC clients and one or more NC servers.

In the context of this application, the term “volume” generally refersto a fixed amount of storage on a storage medium, such as a disk ortape. In some instances, however, the term may be used as a synonym forthe storage medium itself. However, it is possible for a single storagemedium to contain more than one volume or for a volume to span more thanone storage medium.

Network Computer System Overview

In order to obtain the benefits of low-cost administration, all storageof application programs and operating system software for an NC systemis preferably on the NC server. This doesn't mean the NC clients have nomass storage, but rather that the NC clients boot over the network byaccessing a stored copy of the operating system from the NC server.Additionally, when NC clients want to run an application, they access itfrom the NC server. Advantageously, in this manner, an application maybe upgraded by simply replacing the old version of the application witha new version on the NC server. Then, the next time an NC clientrequests the application, the NC client will receive the new version.

FIG. 1 is a simplified block diagram that conceptually illustrates an NCsystem 100 according to one embodiment of the present invention. In thisexample, the NC system 100 includes an NC client 150 coupled to an NCserver 170 via a data communication link 140, such as a 10, 100, or 1000megabit/second Ethernet link. While according to one embodiment, the NCclient 150 is a Macintosh computer or iMac™ computer and the NC server170 is a Mac OS X Server, the NC client 150 and/or the NC server 170 mayalternatively represent one or a combination of the devices/systemsdescribed below with reference to FIG. 2.

The NC server 170 includes one or more system volumes 174, one or moreapplication volumes 176, a user registry 178, non-persistentclient-specific data 184, and persistent user data 186. The systemvolumes 174 include a protected, read-only, master copy of the operatingsystem software. The application volumes 176 include copies of variousapplication programs for use by the NC client 150. According to oneembodiment, the user registry 178 is a database of authorized users,user passwords, NC client hardware addresses, such as Ethernet MediaAccess Control (MAC) addresses, and NC client network addresses, such asInternet Protocol (IP) addresses. In an AppleTalk network environment orthe like, the user registry 178 may comprise the AppleShare Registry.The non-persistent client-specific data 184 represents temporary datastorage that is typically preserved only for the duration of a usersession on a particular NC client. The persistent user data 186represents long-term data storage for user information that is desirableto retain between user sessions, such as preferences, browser bookmarks,and other desktop environment customizations.

The NC server 170 also includes a boot server process 172, a serversoftware management process 180, and a user environment process 182,each of which may include hard-wired circuitry or machine-executableinstructions or a combination thereof. Furthermore, at least a portionof such hard-wired circuitry and/or machine-executable instructions maybe shared between a combination of the boot server process 172, theserver software management process 180, and the user environment process182. In one embodiment, at least one storage area/memory (e.g., amachine-readable medium) having appropriate routines and/or data storedtherein coupled to at least one processor is utilized, at least in part,to implement one or a combination of the boot server process 172, theserver software management process 180, and the user environment process182.

According to one embodiment, the boot server process 172 manages accessto and from the system volumes 174 and the application volumes 176.Additionally, the boot server process 172 performs server-sidebootstrapping processing. In one embodiment, the boot server process 172may include conventional Bootstrap Protocol (Bootp) server processingthat waits for NC clients to appear on the network in accordance withRequest For Comments (RFC) 951, Bill Croft, John Gilmore, BootstrapProtocol, RFC 951, September 1985, RFC 2132, and uses the standardextensions format. As described further below, when the boot serverprocess 172 receives a boot request from the NC client 150, the bootserver process 172 may provide the NC client 150 with a network address,such as an Internet Protocol (IP) address, the address of the NC server170, and the name of a file to be loaded into memory and executed. Thisfirst phase of the bootstrapping process is described by RFC 951 as the“address determination and bootfile selection” phase. The second phaseof the boot strapping process involves the file transfer of the bootfile(also referred to as a boot image) by way of the Trivial File TransferProtocol (TFTP), the Simple File Transfer Protocol (SFTP), the FileTransfer Protocol (FTP), or the like. Further details regarding theparticular information exchanged during these two phases will bedescribed below.

In one embodiment, to facilitate remote NC system administration, suchas the installation of new system or application software, the bootserver process 172 may serve application and system images that exist inthe NC client's folder at the time of bootstrapping as a read-writefile. In this manner, a user having proper access privileges on the NCsystem, such as an NC system administrator, may create a read-write filefrom any NC client in the NC system, modify the read-write file, andsubsequently upgrade the NC system by simply replacing the system and/orapplication volumes on the NC server with the modified file.

In one embodiment the server software management process 180 managesaccess to and from the non-persistent client-specific data 184. Forexample, at the conclusion of a user session on an NC client, the serversoftware management process 180 may provide data that is to be preservedbetween user sessions to the user environment management process 182 andreinitialize the client-specific storage area associated with the NCclient for use by the next user. According to one embodiment, the serversoftware management process 180 performs self-repair functionality attwo levels, automatic and administrator commanded. For example, the NCserver 170 may be configured to automatically replace the shadow volumeat every boot of a NC client 150. This insures that every time that NCclient boots it will have a complete, functional operating systemregardless of any changes any connected user has made to their system.In the event that a user performs some action which would damage theiroperating system in a standard desktop environment, the server softwaremanagement process 180 repairs that damage at the next boot.

According to one embodiment, the user environment management process 182tracks and maintains the persistent user data 186 to insure that changesthe user has made during the current session will be persistent at thenext logic. For example, upon termination of a user session,preferences, desktop items, and various other information representingchanges made by the user during the user session are copies to auser-specific storage location on the NC server 170. Subsequently, atthe user's next login, from the same or different NC client, the userenvironment management process 182 will retrieve the data from thecorresponding user-specific storage location and return it to the NCclient thereby allowing the user to login to any NC client of the NCsystem 100 and have the same user experience.

In one embodiment, the administration tool 188 facilitates NC systemadministration by automating certain file manipulations that are commonfor software installation and configuration changes. The administrationtool 188 is described further below.

While for purposes of illustration, the boot server process 172, theserver software management process 180, the user environment process182, and the administration tool 188 are shown as part of a single NCserver 170, in alternative embodiments, they may be distributed in wholeor in part across multiple server computer systems. Additionally, thesystem volumes 174, the application volumes 176, the user registry 178,the non-persistent client-specific data 184, and the persistent userdata 186 may be independently distributed across multiple servercomputer systems.

The NC client 150 includes one or more system volume images 160 and oneor more application volume images 162. The outlines of the system volumeimages 160 and the application volume images 162 are shown with dottedoutlines because preferably only representations of the content of thecorresponding NC server volumes are stored in random access memory ofthe NC client 150 rather than copies of the actual underlying data andfiles of the corresponding NC server volumes.

The NC client also includes a file system 152, a block device driver154, a network stack 156, and a network device driver 158, each of whichmay include hard-wired circuitry or machine-executable instructions or acombination thereof. Furthermore, at least a portion of such hard-wiredcircuitry and/or machine-executable instructions may be shared between acombination of the file system 152, the block device driver 154, thenetwork stack 156, and the network device driver 158. In one embodiment,at least one storage area/memory (e.g., a machine-readable medium)having appropriate routines and/or data stored therein coupled to atleast one processor is utilized, at least in part, to implement one or acombination of the file system 152, the block device driver 154, thenetwork stack 156, and the network device driver 158.

The file system 152 represents logical structures and software routinesthat are used to control access to and from a local storage medium, suchas a hard disk system. Since the NC client 150 may not have a local massstorage device, the file system 152 may actually control access to andfrom a remote storage medium with or without knowledge that this isoccurring. According to one embodiment, the file system 152 operates asif the contents of the system volumes 160 are, in fact, contained on alocal hard drive and the file system's read and write requests areredirected at a lower level of the operating system. In an alternativeembodiment, the file system 152 itself may be given knowledge regardingthe need to read and write to the network rather than to a local harddrive.

The block device driver 154 services file system read and write requeststo the system volumes 160. In one embodiment, the block device driver154 appears to the file system 152 to be a standard hard drive devicedriver. However, in reality, the block device driver 154 may beconfigured to service the read and write requests of the file system 152by accessing the NC server 170. As will be discussed further below, inone embodiment, one or more shadow volumes, may be created for the NCclient 150 and stored with the non-persistent client-specific data 184for the purpose of storing user preferences and/or changes to theoperating system. Therefore, some mechanism is needed to direct readsand writes to the appropriate volume, e.g., the shadow volumes or thesystem volumes 174. According to one embodiment, the block device driver154 has knowledge of the separate shadow volumes and system volumes 174and redirects read and write requests to the appropriate volume.Alternatively, server-side logic may perform the redirection locally.

The network stack 156 represents logical structures and softwareroutines that are used to control access to and from the datacommunication link 140. In this example, the network stack 156 employsthe services of the network device driver 158 to transform data intoelectrical signals that are appropriate for the specific type ofphysical media, such as Ethernet.

Exemplary Digital Processing System

FIG. 2 is a block diagram of a digital processing system which may beused in accordance with one embodiment of the present invention. Forexample, the digital processing system 200 shown in FIG. 2 may be usedas an NC client, an NC server, or other server computer system, such asan NT server. The digital processing system 200 may be interfaced toexternal systems through a network interface 268. It will be appreciatedthat the network interface 268 may be considered as part of the digitalprocessing system 200. The network interface 268 may be an analog modem,an ISDN modem, a cable modem, a token ring interface, a satellitetransmission interface, a wireless interface, or other interface(s) forproviding a data communication link between two or more digitalprocessing systems.

The digital processing system 200 includes a processor 252, which mayrepresent one or more processors and may include one or moreconventional types of such processors, such as Motorola PowerPCprocessor, an Intel Pentium (or x86) processor, etc. A memory 254 iscoupled to the processor 252 by a bus 256. The memory 254 may be adynamic random access memory (DRAM) and/or may include static RAM(SRAM). The processor may also be coupled to other types of storageareas/memories (e.g., cache, Flash memory, disk, etc.), which could beconsidered as part of the memory 254 or separate from the memory 254.

The bus 256 further couples the processor 252 to a display controller258, an optional mass memory 262, the network interface 268, and aninput/output (I/O) controller 264. The mass memory 262 may represent amagnetic, optical, magneto-optical, tape, and/or other type ofmachine-readable medium/device for storing information. For example, themass memory 262 may represent a hard disk, a read-only or writeableoptical CD, etc. The display controller 258 controls in a conventionalmanner a display 260, which may represent a cathode ray tube (CRT)display, a liquid crystal display (LCD), a plasma display, or other typeof display device. The I/O controller 264 controls I/O device(s) 266,which may include one or more keyboards, mouse/trackball or otherpointing devices, magnetic and/or optical disk drives, printers,scanners, digital cameras, microphones, etc.

It will be appreciated that the digital processing system 200 representsonly one example of a system, which may have many differentconfigurations and architectures, and which may be employed with thepresent invention. For example, Macintosh and Intel systems often havemultiple busses, such as a peripheral bus, a dedicated cache bus, etc.On the other hand, a network computer, which may be used as a digitalprocessing device of the present invention, may not include, forexample, a hard disk or other mass storage device, but may receiveroutines and/or data from a network connection, such as the networkinterface 268, to be processed by the processor 252. Similarly, a Web TVsystem, which is known in the art, may be considered to be a digitalprocessing system of the present invention, but such a system may notinclude one or more I/O devices, such as those described above withreference to I/O device(s) 266. Additionally, a portable communicationand data processing system, which may employ a cellular telephone and/orpaging capabilities, may be considered a digital processing system whichmay be used with the present invention.

The processor 252 may execute one or more routines to redirect read andwrite requests from the file system 152 of the NC client to anappropriate volume on the NC server. Such routines may be stored in themass memory 262, the memory 264, and/or another machine-readable mediumaccessible by the digital processing system 200. According to oneembodiment of the present invention, a network computer (NC) systemmaintains a copy of the operating system in the mass memory 262 (and/orthe memory 254) that cannot be corrupted by ordinary users of the NCsystem. Additionally, the NC system may preserve user customizations,such as preferences, by maintaining individual, user, storage areas inthe mass memory 262 (and/or the memory 254). When an NC client bootsfrom the network and accesses the operating system from an NC server,the user's preferences are dynamically merged with the systemenvironment provided to the NC client. Advantageously, since, the user'sdesktop preferences and other customized settings are all preserved fromsession to session and supplied to the NC client as it boots from thenetwork, the user may login to any NC client on the network and have thesame user experience. According to another embodiment, a networkadministrator can upgrade every NC client in the NC system to a newversion of the operating system by simply replacing a single file on theNC server. Further, according to another feature of the presentinvention, the network administrator can perform such an upgraderemotely from any NC client of the NC system. Advantageously, in thismanner, NC system administration and maintenance costs are kept low ascompared to a typical network of thick clients that each has a localcopy of the operating system that must be replaced.

Net-booting Overview

Having briefly described an exemplary environment in which the presentinvention may be employed, exemplary net-boot processing will now bedescribed with reference to FIG. 3. Providing a reliable, fault-tolerantand maintainable operating system for all NC clients in a net-bootedenvironment is an important component in insuring the successfulimplementation of such as system. NC clients have no operating systeminternally. Therefore, they depend on reliably retrieving a completeoperating system from the network to boot and continue operations.

The NC system environment and the net-booting process described hereinare intended to provide the needed reliability. The net-booting processgenerally breaks down into an address determination and bootfileselection phase, a file transfer phase, a RAM boot phase and a systemboot. The address determination and bootfile selection phase isrepresented by steps 350 through 375, the file transfer phase includessteps 380 and 385, and the RAM boot phase is represented by steps 390and 395. Briefly, for administrative and maintenance purposes,preferably only the NC server 320 has a copy of the master read-onlyoperating system image and NC clients boot according to the net-bootingprocess described below. Since rebooting an NC client restores the NCclient to a standard, useable state, it is impossible for a user withoutproper access privileges to make an NC client unbootable. Consequently,this NC system architecture and net-booting approach greatly simplifiesNC client administration and provides a high level of reliability forthe NC clients.

The first phase of the net-booting processing begins at step 350. Afterthe NC client 310 has powered on, the NC client 310 starts the addressdetermination and bootfile selection phase by initiating the bootprocess out of a local read-only memory (ROM). This may includeexecuting power-on self tests, acquiring device information, andperforming any other functions necessary to give the NC client 310 abasic identify.

At step 355, the NC client 310 connects to the network and asks foradditional information to be provided to it that will allow it to bootfurther. According to one embodiment, the request for additionalinformation takes the form of a Bootp request. In alternativeembodiments, the Dynamic Host Configuration Protocol (DHCP) may beemployed.

At step 360, the NC server 320 receives the boot request and determinesif the request is from a known NC client. For example, the NC server 320may search the user registry 178 for the NC client's hardware address.

If the NC client 310 is unknown to the NC server 320, then an IP addressis allocated for the NC client 310 at step 370 and registrationprocessing is performed. In order to allow the NC server 320 torecognize the NC client 310 next time it boots, information regardingthe NC client 310 may be stored in the user registry 178. For example,the NC client's hardware address can be marked as known in the userregistry 178 and the IP address allocated can be associated in the userregistry with the NC client's hardware address. User registrationprocessing may include creating a user for this NC client 310, e.g., byadding the user to the user registry 178, and creating a private systemfile for the user.

If the NC client 310 is known to the NC server 320, then at step 365information regarding the NC client 310, such as its IP address may beretrieved from the user registry 178.

At step 375, boot information is returned to the NC client 310. The bootinformation may include, among other things, the IP address of the NCclient 310, the IP address of the NC server 320, the name of the file tobe loaded into the NC client's memory and executed, and the names andlocations of shadow volumes, if any.

The second phase of the bootstrapping process, file transfer of thebootfile, begins at step 380. According to one embodiment, the NC client310 sends a file transfer request to the NC server 320 specifying thebootfile identified in the NC server's Bootp reply. At step 385, the NCserver 320 responds by initiating the transfer of the boot image. Uponreceipt at the NC client 310, the boot image is stored in a local RAM ofthe NC client 310.

After the NC client 310 has received the boot image from the NC server320, the RAM boot phase begins at step 390. During the RAM boot phase,the NC client 310 begins to execute the boot image out of the local RAM.According to one embodiment, the boot image is a Macintosh ROM imagethat performs additional machine initialization, and creates a Macintoshenvironment. In alternative embodiments, the boot image may create otheroperating environments, such as Windows 95/98, Windows NT, Windows 2000,etc. In one embodiment, the block device driver 154 may be included aspart of the boot image. Alternatively, the block device driver 154 maybe part of the NC client's ROM.

Finally, at step 395, the NC client 310 mounts the remote system andapplication volumes 174 and 176. Other boot processing (not shown) mayinclude a system boot phase in which the setting up of the operatingenvironment is completed and the virtual memory subsystem isinitialized. At this point, a login may be presented on the NC client310. Based upon the user information, the user environment managementprocess 182 may move all the components that are specific to this user,including preferences, to the user's system environment.

System Read/Write Redirection

As indicated above, the NC client 150 may or may not have a localstorage medium, such as a hard disk system. Therefore, in oneembodiment, the block device driver 154 may redirect read and writerequests received from the file system 152 to a remote storage medium onthe NC server 170, for example.

FIG. 4 is a flow diagram illustrating how NC client write requestsdirected to the system volumes 160 are handled according to oneembodiment of the present invention. In one embodiment, the stepsdescribed below may be performed under the control of a programmedprocessor, such as processor 252. However, in alternative embodiments,the steps may be fully or partially implemented by any programmable orhardcoded logic, such as Field Programmable Gate Arrays (FPGAs), TTLlogic, or Application Specific Integrated Circuits (ASICs), for example.

According to the embodiment depicted, at step 410, the file system 152generates a write request directed to the system volumes 160. The writerequest is received by the block device driver 154 and redirected by theblock device driver 154 at step 420 by translating it into a writerequest directed to the user's shadow volume on the NC server 170. Atstep 430, the NC server 170 stores information associated with the writerequest in the shadow volume associated with the NC client 150. In thismanner, the file system 152 need not be aware that attemptedmodifications to the system volumes 160 are recorded instead in aclient-specific shadow volume residing on the NC server 170.

In alternative embodiments, however, the file system 152 may beconfigured to recognize that write requests should be directed to thenetwork interface 268. In these embodiments, the file system 152 may beconfigured to bypass the block device driver and interface directly withthe network stack 156.

In other embodiments, the block device driver 154 may retain knowledgethat writes are to be redirected to the NC server 170, but may not beaware of the existence of the shadow volume. Therefore, the block devicedriver 154 will issue write requests to the NC server 170, but willissue these requests to the system volumes 174 rather than the shadowvolume. In this situation, logic on the NC server 170 will redirect thewrite requests received from the NC client 150 to the appropriate shadowvolume.

According to yet another alternative embodiment, the NC server 170 maymaintain separate copies of the operating system software in itsentirety for each user. However, this would likely have the effect ofincreasing complexity of the NC server 170 and could potentiallydramatically increase the storage requirements of the NC server 170.

Referring now to FIG. 5, NC client read request processing will now bedescribed according to one embodiment of the present invention. In oneembodiment, the steps described below may be performed under the controlof a programmed processor, such as processor 252. However, inalternative embodiments, the steps may be fully or partially implementedby any programmable or hardcoded logic, such as Field Programmable GateArrays (FPGAs), TTL logic, or Application Specific Integrated Circuits(ASICs), for example.

In the embodiment depicted, it is assumed that the shadow volumecontains only those portions of the operating system that have beenmodified by the user rather than a complete copy of the operating systemas modified. The various options regarding the granularity of theportions written to the shadow volume are discussed below.

At any rate, according the present example, at step 510, the file system152 generates a read request directed to the system volumes 160. At step520, the read request is received by the block device driver 154 and adetermination is made whether the read request specifies a portion ofthe operating system that has been modified by the user or whether theread request specifies a portion of the operating system that remainsunchanged. If the read request corresponds to a portion of the operatingsystem that has not been modified by the user, the processing continueswith step 530. However, if the read request corresponds to a portion ofthe operating system that has been previously modified by the user, thenprocessing continues with step 540.

At step 530, the block device driver directs the NC server 170 toretrieve the information associated with the read request from thesystem volumes 174.

At step 540, the block device driver directs the NC server 170 toretrieve the information associated with the read request from theuser's shadow volume.

In this manner, the file system 152 need not be aware of the particularmechanism used to coordinate user modifications with the originaloperating system as contained in the system volumes 174.

In alternative embodiments, rather than having the block device driver154 keep track of which portions of the operating system that have beenmodified by the user, such a tracking mechanism may be implementedcentrally on the NC server 170. In this case, the NC server 170 wouldmake the determination of step 520 and provide the appropriate readresponses in steps 530 and 540.

In yet another embodiment, the NC server 170 may maintain separatecopies of the operating system in its entirety for each user, includingany user-specific modifications. In this case, neither the block devicedriver 154 nor the NC server 170 would need to track which portion(s) ofthe operating system contain user modifications. Rather, the portion ofthe user-specific copy of the operating system corresponding to the readrequest may simply be returned in response to the read request.

Exemplary Server Directory Structure

FIG. 6 is a simplified block diagram illustrating the layout of anexemplary hierarchical directory structure that may be used by an NCserver according to one embodiment of the present invention.

According to the embodiment depicted, a hard drive 610 of the NC server170 or other mass storage device associated with the NC server 170includes an NC folder (directory) 620. Within the NC folder 620 are anNC admin folder 630 and an NC shared data folder 640. The NC adminfolder 630 is preferably inaccessible to ordinary users that do not haveproper access privileges and includes a read-only, master operatingsystem image 650.

The NC shared data folder 640 includes a shared operating system image660, a boot image 670, and a clients folder 680. The shared operatingsystem image 660 is a read-write version of the read-only masteroperating system image 650 and may be mounted by the NC client 150 aspart of step 395. The boot image 670 is downloaded and executed by theNC client 150 after the first phase of the bootstrapping process inorder to create an operating environment on the NC client 150, such as aMacintosh or Windows environment.

The clients folder 680 is an area that may be used to storenon-persistent client-specific information, such as modifications to theshared operating system 660. In this example, the NC clients arenumbered from 1 to N and the clients folder 680 includes a folder foreach NC client. For example, an NC #1 folder 690 is an area for storingclient-specific information, such as a shadow image 695, correspondingto NC client #1. As described above, the shadow image 695 preferablycontains only portions of the shared operating system image 660 thathave been modified by the user. However, the shadow image 695 may be auser-specific copy of the shared operating image 660 in its entirety.

Network Computer System Administration

As described above, according to one embodiment, NC systemadministration is facilitated by a feature of the boot server process172 that serves application and system images to the NC client 150 as aread-write file if such images exist in the NC client's folder. Thus, auser of the NC client 150 having proper access privileges is providedwith the ability to upgrade the NC system by replacing the existingimages being served to the NC clients with modified versions containing,for example, new system or application software or configurationchanges.

FIG. 7 is a flow diagram illustrating NC system administrationprocessing according to one embodiment of the present invention.According to this example, at step 710, the NC server 170 receives anadministrator logic request from the NC client 150. At step 720, the NCserver 170 performs administrator login processing to verify the userhas appropriate access privileges on the NC system 100. Assuming theuser has appropriate access privileges, processing continues with step730. At step 730, one or more shared images (e.g., shared operatingsystem image 660 and/or a shared application image) are copied to the NCclient's folder on the NC server 170. At step 740, editing of the one ormore shared image copies (the working copies) is enabled. According toone embodiment, this is accomplished by simply rebooting the NC client150 as discussed above. Alternatively, the attributes associated withthe working copies may be modified in some other manner so as to makethem editable by the user. In any event, at step 750, the user may addsoftware and/or make configuration changes to the working copies.Finally, the NC system is updated at step 760 by replacing the one ormore shared images currently being served to NC clients with the workingcopies. Advantageously, in this manner, there is no need to log usersout to insure that changes have been propagated across all the images.Instead, changes become accessible to a NC client the next time that NCclient is booted thereby allowing all connected users to easily completewhatever tasks they are working on without impeding NC systemadministration.

According to one embodiment, steps 730, 740, and 760 may be provided aspart of the administration tool 188. The first execution of theadministration tool 188 may accomplish step 730 by copying the sharedimages to the appropriate NC client folder. After the copies arecomplete, the administration tool 188 may automatically restart the NCclient 150, which will boot up with read-write access to the workingcopies.

At this point, the network administrator is free to add software and/ormake configuration changes. When the network administrator has completedhis/her changes, the administration tool 188 can be run for a secondtime at which point the network administrator may be presented with theoption of either committing (saving) or discarding the changes to theworking copies.

If the administrator chooses to discard the changes, the administrationtool 188 marks the working copies for deletion. Note that immediatedeletion of the working copies is not desirable because the NC client150 is still using the working copies. Rather, it is preferable toremove the working copies that are marked for deletion during asubsequent boot of the NC client 150. In one embodiment, theadministration tool 188 automatically restarts the NC client 150 therebycausing the removal of the working copies and causing the NC client 150to boot from the shared images.

If the administrator chooses to commit the changes, the administrationtool 188 can immediately replace the shared images with those of theworking copies that have been edited which makes the updates immediatelyavailable to any NC client that subsequently boots.

Advantageously, the administration tool 188 allows the networkadministrator to remotely modify the shared images on the NC server 170from an NC client without having to access the NC server 170 directly.Additionally, use of the administration tool 188 allows theadministrator to update the NC system 100 without having to know aboutor directly manipulate the files on the NC server 170.

FIG. 8 is a simplified block diagram illustrating two NC systems 810 and820 coupled via the Internet 805. It is contemplated that the NC systemconfiguration process described above with reference to FIG. 7 may alsobe useful in connection with the reconfiguration of remote NC systems.For example, the administration tool 188 may be run from any of NCclients 812, 814, or 816 to install a new application program on localNC system 810 that is desired by the users of the local NC system 810 aswell as by the users of NC clients 822, 824, and 826 of remote NC system820. After completing the steps described above for installation on thelocal NC system 810, the administration tool 188 running on NC system810 may replicate the modified shared images to the remote NC system820.

Split Operating System

FIG. 9 is a block diagram that conceptually illustrates NC clientinteraction with a SplitOS 920 according to one embodiment of thepresent invention. According to the example depicted, the SplitOS 920 ofthe NC server 170 contains a read-only core system volume image 922 anda read-write user system volume image 924.

The core system volume 922 preferably contains those parts of the systemthat do not need to be written back to during system operation. One goalof the core system volume 922 is to provide all the system componentsthat are mandatory for system operation. By separating these essentialcomponents from the user system volume 924 and protecting them asread-only, an additional level of stability is provided to the NC system100 since a user will be unable to delete or move items that areessential to system operation.

The user system volume 924 contains all the user-configurable systemcomponents, including preferences and all the system additions installedby application software, such as application-installed extensions andlibraries. Additionally, the user system volume 924 may containapplications that cannot be run from an AppleShare server and/or othersystem components that do not function on a read-only volume.

Preferably, the NC server 170 also creates a shadow system volume 930for each connected NC client. The shadow system volume 930 shadows theuser system volume 924 by storing modifications that are made to theuser system volume 924. In alternative embodiments, the NC server 170may provide a separate user system volume 924 for each connected NCclient.

As described above, according to one embodiment, the shadow systemvolume 930 is used by the block device driver 154 of the NC client 150to implement a “copy-on-write” storage scheme in which modifications(writes) directed to the user system volume 930 are instead copied tothe shadow system volume 930. In one embodiment, the shadow systemvolume 930 is not visible to the user. Consequently, the user systemvolume 924 will appear to be the location where all writes are directed.For example, if a user modifies a preference, that preference willappear to be written to the preference file located in the preferencesfolder on the user system volume 924.

As indicated by the directional arrows along the connections between thecore system volume 922, the user system volume 924, and the shadowsystem volume 930 and the NC client 910, data may be read from each ofthe core system volume 922, the user system volume 924, and the shadowsystem volume 930; however, data may only be written to the shadowsystem volume 930.

Shadow System Volume

FIG. 10 is a block diagram that conceptually illustrates the structureof a shadow system volume 1020 according to one embodiment of thepresent invention. In this example, the shadow system volume 1020 is acomplete copy of the system volume 1010 with any user-modificationsincorporated therein. Therefore, the disk space used by the shadowsystem volume 1020 will be approximately the same as that used by thesystem volume 1010.

Banding

FIG. 11 is a block diagram that conceptually illustrates the structureof a sparse shadow system volume 1120 according to another embodiment ofthe present invention in which a banding feature is employed. Accordingto this embodiment, the sparse shadow system volume 1120 is written inband increments rather than as an entire file. A band is a predeterminednumber of blocks. For example, according to one embodiment bands are 128K (256 512-byte blocks). In alternative embodiments, the band size maybe more or less. Using bands, the disk space used by the shadow volume1120 will consist only of the actual data written (in band increments).

According to one embodiment of the banding feature, the inherentone-to-one block mapping in copy-on-write images may be replaced with atable or other data structure that maps a band of the primary file to acorresponding band in the shadow file. Logic may be included in theblock device driver 154, for example, to break up requests into thelargest contiguous chunks possible. In this manner, both reads andwrites can span non-contiguous band boundaries.

The main motivation for banding is to reduce disk space requirements ofthe NC client shadow system images. According to one embodiment, thereduction is achieved by only storing those bands which have actuallybeen written to. An exemplary band table 1122 is a simple arrayallocated at initialization. Entries in the table are assigned BAND_NONEuntil the corresponding band is written to, at which point the nextavailable band in the shadow file is assigned. In order to encouragecontiguous blocks on the NC server's file system, in one embodiment, ifbands are allocated during a write, the shadow file size is increased tothe appropriate number of bands before the write request is sent.

Exemplary Machine-Readable Medium

FIG. 12 is an example of a machine-readable medium 1200 that may beaccessed by a digital processing system, such as an NC client or an NCserver, according to one embodiment of the present invention.Importantly, the actual memory that stores the elements shown in anddescribed below with reference to FIG. 12 may comprise one or more disks(which may, for example be magnetic, optical, magneto-optical, etc.),the memory 254 and/or the mass memory 262 described above with referenceto FIG. 2, or a combination thereof. Additionally, one or more of theelements of the machine-readable medium 1200 may be stored at onedigital processing system, such as an NC server, and downloaded ortransmitted to another digital processing system, such as an NC clientor another NC server. Furthermore, the elements described with referenceto the machine-readable medium 1200 may, at some point in time, bestored in a non-volatile mass memory (e.g., a hard disk). Conversely, atother times, the elements of the machine-readable medium 1200 may bedispersed between different storage areas, such as DRAM, SRAM, disk,etc.

According to one embodiment, the machine-readable medium 1200 isutilized, at least in part, to facilitate administration and maintenanceof system and/or application volumes of an NC system in accordance withone or more methods of the invention. For example, the machine-readablemedium 1200 may be associated with an NC server and include one or moreroutines for performing bootstrapping as discussed with reference toFIG. 3 (the boot server program 1205), for performing system read andwrite processing as discussed with reference to FIGS. 4 and 5 (the blockdevice driver 1230), and/or for performing remote administration of theNC system 100 as discussed with reference to FIG. 7 (the administrationprogram).

While the block device driver 1230 is show as being separate from theboot image 1210 in this example, the block device driver may be providedas part of the boot image 1210 in alternative embodiments.

Alternative Embodiments

While the invention has been described with reference to specificembodiments and illustrative figures, those skilled in the art willrecognize that the invention is not limited to the embodiments orfigures described. In particular, the invention can be practiced inseveral alternative embodiments that provide net-booting and remoteadministration of an NC system.

Therefore, it should be understood that the method and apparatus of theinvention can be practiced with modification and alteration within thespirit and scope of the appended claims. The description is thus to beregarded as illustrative instead of limiting on the invention.

1. A computer implemented method comprising: a network computer (NC)client booting from a boot image provided by an NC server, the bootimage including information identifying a location of one or more systemvolumes on the NC server, the one or more system volumes containingoperating system software to establish an operating environment for theNC client; and in response to an attempt to modify contents of the oneor more system volumes, the NC client causing information identifying amodification associated with the attempt to be recorded on the NC serverseparate from the one or more system volumes in a storage areaassociated with the NC client, wherein the recorded informationidentifying the modification is stored in a separate volume separatedfrom the one or more system volumes, wherein the separate volume ispartitioned into a plurality of bands, each band having a predeterminednumber of blocks, wherein the separate volume is written in a bandincrements rather than as an entire file, wherein a modification to afile in the one or more system volumes is partitioned into one or morebands and only those bands that have actually been modified are storedin one or more bands of the separate volume, and wherein the modifiedbands of the file in the one or more system volumes and thecorresponding one or more bands of the separate volume are mapped by aband table.
 2. The method of claim 1, further comprising: transmittinginformation identifying a user of the NC client to the NC server;receiving information identifying the user's desktop environmentpreferences from the NC server; and customizing a desktop environment ofthe NC client in accordance with the user's desktop environmentpreferences.
 3. The method of claim 1, wherein the one or more systemvolumes are presented to the NC client as a split operating systemincluding a core operating system volume that can be read but notwritten by the NC client and the user operating system volume that canbe read and/or written by the NC client, wherein the storage areaassociated with the NC client comprises a shadow volume corresponding tothe user operating system volume, and wherein the NC client causinginformation identifying the modification associated with the attempt tobe recorded comprises tracking modifications to the user operatingsystem volume in the shadow volume.
 4. The method of claim 1, furthercomprising, prior to booting from the boot image provided by the NCserver, (1) the NC client initiating a boot process by booting into alocal memory of the NC client, (2) the NC client transmitting a bootrequest to the NC server, and (3) the NC client receiving the boot imagefrom the NC server.
 5. The method of claim 3, wherein booting from theboot image provided by the NC server further includes the NC clientlocally executing the boot image and mounting the one or more systemvolumes.
 6. A network computer (NC) client comprising: a bootstrappingmeans for booting from a boot image provided by an NC server, the bootimage including information identifying a location of one or more systemvolumes on the NC server, the one or more system volumes containingoperating system software to establish an operating environment for theNC client; and a redirecting means, responsive to an attempt to modifycontents of the one or more system volumes, for causing informationidentifying a modification associated with the attempt to be recorded onthe NC server separate from the one or more system volumes in a storagearea associated with the NC client, wherein the recorded informationidentifying the modification is stored in a separate volume separatedfrom the one or more system volumes, wherein the separate volume ispartitioned into a plurality of bands, each band having a predeterminednumber of blocks, wherein the separate volume is written in a bandincrements rather than as an entire file, wherein a modification to afile in the one or more system volumes is partitioned into one or morebands and only those bands that have actually been modified are storedin one or more bands of the separate volume, and wherein the modifiedbands of the file in the one or more system volumes and thecorresponding one or more bands of the separate volume are mapped by aband table.
 7. The NC client of claim 6, further comprising a bandingmeans for incorporating the modification within one or more bandscomprising a predetermined number of blocks.
 8. A computer-implementedmethod comprising: a network computer (NC) client booting from a bootimage provided by an NC server, the boot image including informationidentifying a location of one or more system volumes on the NC server,the one or more system volumes containing operating system software toestablish an operating environment for the NC client; the NC clientmounting the one or more system volumes; and in response to a writerequest from a file system of the NC client that contains a modificationto the one or more system volumes, a block device driver of the NCclient redirecting the write request and causing information identifyingthe modification to be recorded on the NC server in a storage areaassociated with the NC client that is separate from the one or moresystem volumes, wherein the recorded information identifying themodification is stored in a separate volume separated from the one ormore system volumes, wherein the separate volume is partitioned into aplurality of bands, each band having a predetermined number of blocks,wherein the separate volume is written in a band increments rather thanas an entire file, wherein a modification to a file in the one or moresystem volumes is partitioned into one or more bands and only thosebands that have actually been modified are stored in one or more bandsof the separate volume, and wherein the modified bands of the file inthe one or more system volumes and the corresponding one or more bandsof the separate volume are mapped by a band table.
 9. Acomputer-implemented method comprising: a network computer (NC) clientbooting from a boot image provided by an NC server, the boot imageincluding information identifying a location of one or more systemvolumes on the NC server, the one or more system volumes containingoperating system software that has one or more customizable attributes;and in response to a change to an attribute of the one or morecustomizable attributes, the NC client causing information identifyingthe change to be recorded on the NC server in a storage area associatedwith the NC client that is separate and distinct from the one or moresystem volumes, wherein the recorded information identifying themodification is stored in a separate volume separated from the one ormore system volumes, wherein the separate volume is partitioned into aplurality of bands, each band having a predetermined number of blocks,wherein the separate volume is written in a band increments rather thanas an entire file, wherein a modification to a file in the one or moresystem volumes is partitioned into one or more bands and only thosebands that have actually been modified are stored in one or more bandsof the separate volume, and wherein the modified bands of the file inthe one or more system volumes and the corresponding one or more bandsof the separate volume are mapped by a band table.
 10. Acomputer-implemented method comprising: a network computer (NC) serverproviding a boot image to an NC client, the boot image includinginformation identifying a location on the NC server of one or moresystem volumes containing operating system software to establish anoperating environment for the NC client; and in response to a writerequest from the NC client that contains a modification to the operatingsystem software, the NC server recording information identifying themodification on the NC server in a storage area associated with the NCclient that is separate from the one or more system volumes, wherein therecorded information identifying the modification is stored in aseparate volume separated from the one or more system volumes, whereinthe separate volume is partitioned into a plurality of bands, each bandhaving a predetermined number of blocks, wherein the separate volume iswritten in a band increments rather than as an entire file, wherein amodification to a file in the one or more system volumes is partitionedinto one or more bands and only those bands that have actually beenmodified are stored in one or more bands of the separate volume, andwherein the modified bands of the file in the one or more system volumesand the corresponding one or more bands of the separate volume aremapped by a band table.
 11. The method of claim 10, further comprisingthe NC server maintaining the one or more system volumes as a splitoperating system including a single core operating system volume thatcan be read but not written by the NC client and a user operating systemvolume that can be both read and written by the NC client.
 12. Themethod of claim 11, wherein the storage area associated with the NCclient contains a non-persistent shadow volume corresponding to the useroperating system volume to which modifications to the user operatingsystem volume are recorded.
 13. The method of claim 12, furthercomprising storing information from the shadow volume to a persistent,user-specific storage area for use in a subsequent user session.
 14. Themethod of claim 13, further comprising: receiving informationidentifying the user of the NC client; and providing the NC client withinformation indicative of the user's desktop environment by accessingthe persistent, user-specific storage area.
 15. A network computer (NC)server comprising: a boot server means for providing a boot image to anNC client, the boot image including information identifying a locationon the NC server of one or more system volumes containing operatingsystem software to establish an operating environment for the NC client;and a storage management means for recording information identifying amodification to the operating system software in a storage areaassociated with the NC client that is separate from the one or moresystem volumes, the storage management means operative in response to awrite request from the NC client that contains the modification, whereinthe recorded information identifying the modification is stored in aseparate volume separated from the one or more system volumes, whereinthe separate volume is partitioned into a plurality of bands, each bandhaving a predetermined number of blocks, wherein the separate volume iswritten in a band increments rather than as an entire file, wherein amodification to a file in the one or more system volumes is partitionedinto one or more bands and only those bands that have actually beenmodified are stored in one or more bands of the separate volume, andwherein the modified bands of the file in the one or more system volumesand the corresponding one or more bands of the separate volume aremapped by a band table.
 16. A machine-readable storage medium havingstored thereon data representing sequences of instructions, thesequences of instructions which, when executed by a processor, cause theprocessor to perform: providing a boot image to a network computer (NC)client, the boot image including information identifying a location onan NC server of one or more system volumes containing operating systemsoftware to establish an operating environment for the NC client; and inresponse to a write request from the NC client that contains amodification to the operating system software, recording informationidentifying the modification in a storage area associated with the NCclient that is separate from the one or more system volumes, wherein therecorded information identifying the modification is stored in aseparate volume separated from the one or more system volumes, whereinthe separate volume is partitioned into a plurality of bands, each bandhaving a predetermined number of blocks, wherein the separate volume iswritten in a band increments rather than as an entire file, wherein amodification to a file in the one or more system volumes is partitionedinto one or more bands and only those bands that have actually beenmodified are stored in one or more bands of the separate volume, andwherein the modified bands of the file in the one or more system volumesand the corresponding one or more bands of the separate volume aremapped by a band table.
 17. In a network computer (NC) system, a methodcomprising: an NC server providing a boot image to an NC client, theboot image including information identifying a location on the NC serverof one or more system volumes containing operating system software toestablish an operating environment for the NC client; the NC clientbooting from the boot image provided by the NC server; the NC clientmounting the one or more system volumes; in response to a write requestfrom a file system of the NC client that contains a modification to theone or more system volumes, a block device driver of the NC clientredirecting the write request to a storage area on the NC server that isassociated with the NC client and which is separate from the one or moresystem volumes; the NC server receiving the write request from the NCclient; and the NC server causing information identifying themodification to be recorded in the storage area associated with the NCclient, wherein the recorded information identifying the modification isstored in a separate volume separated from the one or more systemvolumes, wherein the separate volume is partitioned into a plurality ofbands, each band having a predetermined number of blocks, wherein theseparate volume is written in a band increments rather than as an entirefile, wherein a modification to a file in the one or more system volumesis partitioned into one or more bands and only those bands that haveactually been modified are stored in one or more bands of the separatevolume, and wherein the modified bands of the file in the one or moresystem volumes and the corresponding one or more bands of the separatevolume are mapped by a band table.
 18. A network computer (NC) systemcomprising: an NC server configured to provide a boot image to one ormore NC clients associated with the NC system, the boot image includinginformation identifying a location on the NC server of one or moresystem volumes containing operating system software to establish anoperating environment for the NC client; and an NC client coupled incommunication with the NC server, the NC client configured to receiveand boot from the boot image, the NC client including a file systemprocess and a block device driver, the block device driver configured toredirect write requests directed to the one or more system volumes to astorage area on the NC server that is associated with the NC client andwhich is separate from the one or more system volumes, wherein therecorded information identifying the modification is stored in aseparate volume separated from the one or more system volumes, whereinthe separate volume is partitioned into a plurality of bands, each bandhaving a predetermined number of blocks, wherein the separate volume iswritten in a band increments rather than as an entire file, wherein amodification to a file in the one or more system volumes is partitionedinto one or more bands and only those bands that have actually beenmodified are stored in one or more bands of the separate volume, andwherein the modified bands of the file in the one or more system volumesand the corresponding one or more bands of the separate volume aremapped by a band table.